The present invention relates to a spherical annular seal used in a spherical pipe joint for an automobile exhaust pipe and a method of manufacturing the same.
Conventionally, a spherical annular seal for use in a spherical pipe joint for an automobile exhaust pipe, in which a mixture of relatively short fibers (2 to 8 mm) and a solid lubricant such as graphite is compression-formed, is known, as disclosed in, for instance, U.S. Pat. No. 4,659,091 (hereafter referred to as the prior art publication 1).
Although the seal disclosed in this prior art publication 1 has the advantage that it can be manufactured easily at low cost, the seal has a drawback in that stick slip often generating abnormal noise can be induced in friction with a mating member. Also, the seal has a problem in that, owing to problems with materials constituting the seal and with a method of manufacturing the seal, the strength of the seal comes to decline over periods of use, possibly resulting in partial fracture or breakage of the seal and thereby lowering the function of the seal.
The seal in accordance with the prior art publication 1 has a spherical annular shape, and is provided with a through hole in its center and a partially convex spherical surface portion on its outer side. Therefore, particularly at the time of manufacturing the seal, portions filled coarsely with material and other portions filled densely with material can occur in a small-diameter portion (a tip portion of the seal) of the partially convex spherical surface portion of the seal. In that case, pressure cannot be applied sufficiently to the coarsely filled portions during subsequent compression forming, so that an insufficient-strength portion may occur in that portion. The insufficient-strength portion possibly leads to fracture or breakage of that portion during the use of the seal, causing leakage of exhaust gases.
Accordingly, in view of the problem of a decline in the strength of the seal in accordance with the above-described prior art publication 1 and the problem of occurrence of abnormal noise during an initial period of friction with the mating member, a seal disclosed in Japanese Patent Unexamined Publication No. 215916/1990 (hereafter referred to as the prior art publication 2) has been proposed.
Namely, the seal proposed in the prior art publication 2 is arranged such that a mixture including a solid lubricant, such as graphite, metal fibers 10 to 200 .mu.m in diameter and 100-800 .mu.m in length, and a resin binder is so compressed together with a tubular reinforcing member formed of a metal mesh that the mixture is filled in mesh openings of the tubular reinforcing member and surrounds the reinforcing member, a through hole with the reinforcing member exposed being formed in its center and a partially convex spherical surface portion being formed on its outer surface, wherein a lubricating sliding layer including tetrafluoroethylene or a lubricating composition having tetrafluoroethylene as a main component is formed on the partially convex spherical surface portion of the seal.
In accordance with the prior art publication 2, it is possible to obtain a seal having enhanced strength without causing insufficiency in strength attributable to the coarse and dense filling of the material in the small-diameter portion of the partially convex spherical surface portion, which is the drawback of the above-described prior art publication 1. In addition, since the lubricating sliding layer is formed on the surface of the partially convex spherical surface portion which is a frictional surface in friction with a mating member, the drawback of the occurrence of abnormal noise particularly during an initial period of friction is not experienced. Thus, the problems of the seal in accordance with the prior art publication 1 are overcome.
It is true that the seal in accordance with the above-described prior art publication 2 overcomes the drawbacks of the seal disclosed in the prior art publication 1 according to the test conditions and test method disclosed in the publication 2. However, in a case where the seal is used under test conditions and a test method which are severer than those of the aforementioned test conditions and test method, there arises a new problem of an exfoliation or peeling taking place in the lubricating sliding layer formed on the partially convex spherical surface portion on its outer surface, resulting in the occurrence of abnormal noise.
In particular, as for the spherical pipe joint used for the purpose of attenuating vibrations transmitted from the automobile engine to the exhaust pipe, the position of its installation in the exhaust pipe extending from the engine to the muffler is determined by taking various conditions into consideration. Accordingly, the seal which is incorporated in such a joint is required to be capable of coping with all the conditions.